In the Ebb and Flow Of Ancient Glaciers, Clues to Next Ice Age

By WILLIAM K. STEVENS

Published: January 16, 1990

THE climatic history of the last half-million years is largely a history of ice and cold, punctuated every 100,000 years or so by brief, glorious intervals of unusual warmth called interglacials. If nature takes its course, scientists say, the current interglacial - the one in which human civilization developed - should begin to end within the next 2,000 years or sooner. But nature may not take an unaltered course. Unprecedented concentrations of heat-trapping carbon dioxide in the atmosphere could extend the present interglacial by a millennium or more and transform it into a ''super-interglacial'' with average global temperatures higher than any seen in the last million years, say a number of scientists who study the advance and retreat of the great glaciers.

Putting off the next ice age might seem an attractive prospect; in the depths of the last one, 18,000 years ago, global temperatures averaged almost 10 degrees colder than today's and much of North America was buried under ice averaging a mile thick. Some experts say, however, that such a delay carries its own risks and uncertainties. There has never been more carbon dioxide in the air than now, they say, and since the burning of fossil fuels is causing the concentration to rise with no end to the increase in sight, there is no reliable guide to the climatic changes and disruptions it might cause.

''We're going outside what nature has experienced in the recent past,'' said Dr. Nicholas Shackleton of Cambridge University in England, who has long been in the forefront of research on the glacial periods. ''It's scary.''

In the last decade or so, scientists have made great strides in understanding the causes and workings of the great glacial cycles. But how much the globe will warm up is a matter of intense argument, and some glaciologists who believe that a super-interglacial is on the way say it may not last long enough to delay the world's turn to a new ice age. How all this will play out is one of the major questions faced by the interrelated sciences of glaciology and climatology.

The present interglacial is 10,000 to 13,000 years along, and scientists say that in at least the last half-million years, no interglacial has ever lasted longer than 12,000 years. Over the last few years, scientists have generally come to agree on what determines the timing of these warm cycles and of the long, uneven cold spells that culminate in full ice ages.

The glacial cycles are set in motion by periodic wobbles in the Earth's rotation and changes in the tilt of its axis and the shape of its orbit, occurring over tens of thousands of years. The three overlapping cycles control the timing of global warming and cooling, and the long-term advance and retreat of the glaciers, by altering the angles and distances from which the Sun's energy strikes the Earth.

The shortest cycle, which is repeated mainly in periods of 19,000 and 23,000 years, is that of the wobble in the Earth's rotation. The wobble, or precession, is much like that of a spinning top as it loses speed. In the second cycle, the tilt of the Earth's axis as it orbits the Sun varies from about 22 degrees to about 25 degrees over 41,000 years. In the longest cycle, the Earth's orbit stretches slightly from nearly circular to elliptical and back again every 100,000 years or so.

To some degree, said Dr. John Imbrie of Brown University, a leading glaciologist, the two shorter cycles appear to exert a direct force on the climate, much as the Earth's movement about the Sun exerts a direct influence on the change of seasons. By itself the 100,000-year cycle does not appear strong enough to bring about direct alterations in the Earth's climate. But scientists believe it sets off complex and far-reaching changes in the ocean-atmosphere system that drives the climate.

This astronomical theory of glaciation, as it is called, was first set forth in the 1920's by Milutin Milankovitch, a Yugoslav mathematician, but confirmed only in the 70's and 80's. Researchers did so by analyzing ocean sediment in which the glacial cycles were reflected in the remains of plankton-like animals that live at particular temperatures, and by studying the ancient distribution of isotopes of oxygen whose proportions vary with the temperature.

The focus of research now is on understanding how all three cycles affect the workings of the dauntingly complex ocean-atmosphere system and how they will be affected by rising concentrations of carbon dioxide and other greenhouse gases.

Ice ages and interglacials differ from each other in their duration and internal fluctuations, partly because the interaction of the three astronomical cycles is rarely the same. The ice ages cannot be forecast exactly; Dr. Imbrie said they have ''an average recurrence time.''

Coming: Global Cooling

Scientists calculate that in the normal course of the cycle, apart from any warming induced by humans, the present interglacial should give way within no more than 2,000 years to a gradual, uneven decline in global temperature that would bring moderate cold about 5,000 years from now and major ice ages about 23,000 and 60,000 years from now.